Dennis D. McCarthy, who is paid $25,573 a year as a dogcatcher for Montgomery County, did only 30 minutes of work during his eight-hour shift yesterday.

That half hour of "shuffling paper" was an unusually busy stint for him, said McCarthy. Since at least March 1982, he acknowledges, he has refused to perform most of his official duties.

The reason: He was illegally transferred from another county job, he says, and working with animals has given him high blood pressure, a skin rash and emotional stress.

"I am not doing any work for this dumb office," he said yesterday. "I will spend my time reading my books or listening to my music."

McCarthy's refusal to work has become a major embarrassment for the administration of County Executive Charles W. Gilchrist.

But the county is about to act. Officials say medical tests show McCarthy has little if any adverse reaction to animals and he will be asked today either to buckle down to his job or quit.

In a series of recent interviews, officials ranging from McCarthy's supervisor to aides to chief administrative officer Lewis T. Roberts acknowledged that McCarthy should have been fired or transferred long ago.

"I made a mistake in not coming down quicker on him," said David E. Wheeler, the county animal control director.

"It's an embarrassment for the people I work for and an embarrassment to my department, which bothers me even more."

Wheeler has directed McCarthy to make the required rounds of animal calls and do the resulting paper work since 1980, when, during a series of layoffs, McCarthy was transferred from his job as a county building inspector.

But in that time--years punctuated by employment grievances he filed against the county--McCarthy refused to do the required work, he and other officials said. "I was put into that job against my will and in violation of the regulation requiring employes to be qualified for the work they do," said McCarthy, a feisty, 61-year-old Canadian. "Working with animals is psychologically upsetting. They can go to hell," McCarthy added. "I have no intention of doing that job."

That sentiment hardened earlier this summer when Thomas B. Ferguson, a county management expert assigned to the animal control department, learned that McCarthy spent all but a fraction of his work days reading novels and listening to classical music.

When McCarthy complained that working with animals made him ill, officials ordered him to undergo medical tests. The results are expected to be released today.

Administration sources familiar with McCarthy's case said they will show little if any evidence that working with animals makes McCarthy physically ill.

"If he is found to be fit for duty and not at risk medically, he will be ordered to perform his job," said Ferguson.

"If he is at risk, we will attempt to make a reasonable accommodation" and transfer him, he said.

"But if he refuses to obey a direct order to work," Ferguson added, "he will face disciplinary action up to and including dismissal."

There may be at least one other casualty in this episode, according to officials familiar with it.

As McCarthy's supervisor, Wheeler could face administrative punishment for allowing McCarthy not to do his job, officials and McCarthy said.